Everything is clear now, thank you!
However, in the thread I was referring to you can also find the sentence *tlhIngan vIjatlh* ("I speak to the Klingon").
I don't see that in the thread you linked to. I see tlhIngan Hol vIjatlh, not tlhIngan vIjatlh.
Would that be an instance of the "prefix trick"?
No, that would probably be considered an error. Someone probably forgot to add Hol to indicate that they were speaking "Klingon language."
The sentence is in another message of the same thread, here: http://lists.kli.org/pipermail/tlhingan-hol-kli.org/2022-June/063068.html
Are there other verbs apart from *ja'* (and maybe *jang*) that can take an object with the semantic role of the indirect object? Probably lots of them. ghojmoH is an example. In paq'batlh we see both [puqloDwI'] vIghojHa'moH and QIt ghaHvaD yIn Hegh je vIghojmoH. In the former, the student is the object; in the latter life and death is the object in the role of direct object, and he is the beneficiary in the role of indirect object.
*tlhIngan Hol DaghojmoH* ("You teach Klingon") - No "prefix trick" *'eSpanya'ngan DaghojmoH* ("You teach the Spaniard") - No "prefix trick" *DaghojmoH* (depending on context "You teach it/them" or "You teach her/him/them") - No "prefix trick" *choghojmoH* ("You teach me") - No "prefix trick" *jIHvaD tlhIngan Hol DaghojmoH* ("You teach me Klingon") - No "prefix trick" *tlhIngan Hol choghojmoH* ("You teach me Klingon") - "Prefix trick" Right?